1. Field of the Invention
The present invention relates to a control apparatus of an extracted air booster system of an integrated gasification combined cycle power plant. Specifically, the present invention relates to control of degrees of opening respectively of an inlet guide vane and an antisurge valve (pressure control and antisurge control) of the extracted air booster system.
2. Description of the Related Art
An integrated gasification combined cycle power plant is a new type of power plant which is being currently developed for commercialization. A feature of such an integrated gasification combined cycle power plant is that a gasification system and a gas turbine system, which are principal components thereof, supply gas and air to each other.
Accordingly, an integrated gasification combined cycle power plant is provided with an extracted air booster system. In the extracted air booster system, air (compressed air) which is extracted from a compressor of a gas turbine system is further pressurized by an extracted air booster, and then the pressurized air is sent to a gasification furnace of a gasification system.
The extracted air booster system includes the extracted air booster, an inlet guide vane and an antisurge valve (recirculation valve). The extracted air booster pressurizes air extracted from the compressor driven by a gas turbine of the gas turbine system, and supplies the extracted air thus pressurized to the gasification furnace. The inlet guide vane is provided to an inlet side of the extracted air booster, and the antisurge valve returns the extracted air from an outlet side of the extracted air booster to the inlet side of the extracted air booster.
In order to set the pressure of the extracted air to be supplied to the gasification furnace at a predetermined pressure, and to perform surge protection for the extracted air booster, a control apparatus of the extracted air booster system performs pressure control and antisurge (surge prevention) control. In the pressure control, based on a pressure deviation between a set pressure value and an outlet pressure detection value of the extracted air booster, the degree of opening of the inlet guide vane is controlled so that the outlet pressure detection value can become equal to the set pressure value. In the antisurge control, based on a pressure ratio deviation between a set pressure ratio value and a pressure ratio of the outlet pressure detection value of the extracted air booster to an inlet pressure detection value of the extracted air booster, the degree of opening of the antisurge valve is controlled so that the pressure ratio can become equal to the set pressure ratio value.
The extracted air booster is an apparatus for pressurizing air extracted from the compressor of the gas turbine system to a predetermined pressure suitable for supplying the extracted air to the gasification furnace. Since the gasification furnace is operated at a constant pressure, the control apparatus of the extracted air booster system performs pressure control so that the outlet pressure of the extracted air booster can become equal to the set pressure by controlling the degree of opening of the inlet guide vane, as described above. Moreover, since the gasification furnace is operated at a constant pressure, the pressure ratio of the extracted air booster increases and comes closer to a surge region, as a load decreases. Accordingly, a difficult operating situation occurs in the extracted air booster. To cope with this, the control apparatus of the extracted air booster system controls the degree of opening of the antisurge valve as described above to perform antisurge control so that the pressure ratio of the extracted air booster can become equal to the set pressure ratio.
Prior art documents which disclose a method and the like of controlling an extracted air booster system of such an integrated gasification combined cycle power plant include, for example, Japanese Unexamined Patent Application Publication No. H5-248260 and Japanese Unexamined Patent Application Publication No. H9-96227.
However, the control of an extracted air booster system of an integrated gasification combined cycle power plant has not yet been sufficiently established.
To be more detailed, in a conventional system or the like in which natural gas is pressurized by a gas compressor to be supplied to a gas turbine, the capacity of the gas compressor is not tight. Accordingly, the control of a recirculation valve is the main operation, and an inlet guide vane is operated only during a high-load period in which the recirculation valve is fully closed. However, an extracted air booster system of an integrated gasification combined cycle power plant does not have enough margin of operation because of its small system capacity. For this reason, the inlet guide vane needs to be actively operated to enlarge the control range.
For this reason, as described previously, the following role sharing has been employed: an inlet guide vane is used for a pressure control function, and an antisurge valve (recirculation valve) is used for an antisurge control function. However, there have been problems including the inlet guide vane being incapable of serving a sufficient pressure control function by itself.